1. Field of the Invention
The present invention relates to a static power switch incorporating semi-conductors.
2. History of the Related Art
FIG. 1 of the accompanying drawings schematically shows in much simplified manner a unidirectional static circuit breaker of the prior art. In this example, the load 1 is supplied by a D.C. voltage source 2 and the load is connected in series with a power switch 3 of "GTO" type (i.e. blockable by the gate).
A current sensor 4 applies an image signal of the line current on an input 5 for tipping the control circuit 6 of the thyristor 3. In the event of accidental overintensity, the tipping threshold of the circuit 6 is exceeded and the circuit controls, by the gate 7, the extinction of the thyristor 3, which cuts the supply to the load 1.
The static switch constituted by the thyristor 3 can, however, in theory and in practice, not operate without there being provided at its terminals a switching aid circuit 8, more commonly known as a "snubber" circuit, whose essential role is to limit the rise in the voltage at its terminals at the moment of opening of switch 3.
This snubber circuit 8, which conventionally includes a capacitor 9 placed in series with a discharging resistor 10 shunted by a diode 11. The snubber circuit operates in the following manner:
Upon opening of the switch 3, the self-inductance of the load 1 naturally opposes the decline in the line current which circulates in the load. At first approximation, the current therefore remains constant immediately after opening, while, in the absence of a snubber circuit, the voltage at the terminals of the switch 3 (which was zero before the opening) undergoes a sudden increase, causing at the terminals of the switch an excess voltage which is generally prohibitive and destructive.
The snubber circuit 8 being present, a capacitor 9 is connected to the terminals of the switch 3, with the capacitor's load being zero as long as the switch is closed.
Upon opening, the line current remains, as mentioned above, virtually constant. As the current in the switch 3 undergoes an approximately linear decline, it follows that the charging current of the capacitor 9 undergoes the reverse linear law, with the result that the voltage at its terminals (and consequently the voltage at the terminals of the switch 3), which follows a law of increase which is the integral of the latter linear law of variation of the current, finally varies in accordance with a parabolic law. This voltage therefore rises very slowly, with the result that the losses upon opening in the thyristor 3 are acceptable.
The role of resistor 10 is obviously to limit the intensity of the discharging current of the capacitor 9 upon closure of the switch 3, while diode 11 naturally shunts the resistor upon charging at opening.
In the case of high-power circuits, for instance when the maximum overintensity admissible in the load is for example of the order of 800 amps, the thyristor 3 is conventionally chosen accordingly from those normally available on the market. This results in a remarkably high cost and space requirement.
If it is now desired to be able to switch an accidental overintensity of, for example, 1200 amps, two large thyristors of the above type will conventionally be used, which will then be wired in parallel, with one snubber circuit per thyristor. The drawbacks of high cost and space requirement therefore remain.